Various devices including aircraft and guided missiles which travel at high velocities are controlled by transmitting a signal from a remote station to an infrared (IR) sensor or window located on-board the device. While in operation, the IR sensor or window is exposed to considerable heat loading and erosion due to impact of particles. Such exposure oftentimes exceeds the working capabilities of the IR sensor or window. Even the smallest atmospheric dust particles can scratch the IR sensor or window; and over time, cause considerable erosion effects on the optical transmissivity of the IR sensors or windows. The term “optical transmissivity” is used herein to refer to the ability a material has to allow desired wavelengths of radiate energy, or light, to pass through it.
Materials which can be used to make IR sensors or windows include, but are not limited to: zinc sulfide (ZnS), zinc selenide (ZnSe), germanium (Ge), silicon (Si), gallium arsenide (GaAs), gallium phosphide (GaP), fused silica, aluminum oxynitride (AlON), sapphire (Al2O3), magnesium oxide (MgO), spinel (MgO—Al2O3), cubic zirconia (c-ZrO2), lanthana-doped yttria, yttria (Y2O3), mixed fluoride glasses and other optical transmissive materials. These optical transmissive materials are generally temperature sensitive materials (i.e., they have a low softening temperature) that oftentimes fail due to thermal shock caused by atmospheric friction at high velocities. Additionally, optical transmissive materials are generally soft materials and therefore damage easily upon use.
Protective coatings and films are typically applied to protect optical transmissive materials from damage caused by thermal shock and erosion/abrasion. One coating that has been successfully employed in protecting optical transmissive materials is a hard carbon film that has diamond-like properties, e.g., a diamond-like carbon (DLC) film. However, DLC films often require high-temperatures and atomic hydrogen for deposition, both of which can degrade the optical transmissive material unless various interlayers are employed. Unfortunately, suitable interlayers are difficult to find and oftentimes the interlayer delaminates at high-temperatures, further complicating the process.
Furthermore, interlayers and DLC coatings may interfere with the high degree of optical transmissivity often required for such devices. To be useful, any protective coating, or interlayer for use with optical transmissive materials, must itself be highly optically transmissive. The optical transmissivity of the coating or interlayer itself must also be able to withstand high-operating temperatures.
Another known coating material is germanium-carbon. Germanium-carbon is a hard, amorphous material containing Ge, C and H, see, for example, A. H. Lettington, et al., Proc. SPIE 1112, 156–61 (1989), and J. M. Mackowski, et al., Proc. SPIE 1760, 201–9 (1992). Germanium-carbon can be made from GeH4 and C4H10 or CH4 by PECVD (at temperatures of about 350° C. or above), or by sputtering a Ge target in a hydrocarbon atmosphere. Because of relatively low intrinsic stress germanium-carbon can be grown in thick layers (>100 microns), but provides only modest rain erosion resistance.
The refractive index of germanium-carbon can be varied between 2 and 4 by changing the Ge:C ratio. The absorption coefficient of coatings deposited using one method was less than 10 cm−1 in the 3–12 micron range. A more Ge-rich preparation had an absorption coefficient in the 40–270 cm−1 range at 10.6 microns. Young's modulus for germanium-carbon is reported near 300 GPa with a nanoindentation hardness in the 14–20 GPa range. An abrasion-resistant, multilayer, dual-band (3–5 and 8–12 micron) antireflection coating on ZnS is based on several different layers of germanium-carbon.
Although germanium-carbon films are known, such films are typically formed at high-temperatures, which cause the film to delaminate from the IR transmissive material and/or effect the optical transmissivity of the IR transmissive material.
In view of the state of the art mentioned above, there is a need for providing a coating for protecting temperature sensitive optical materials, which is highly resistant to abrasion, yet is capable of being optically transmissive itself. Such an abrasion-resistant coating should be applied in a manner that does not adversely affect the transmissivity of the underlying optical material.